In some quick-plastic-forming processes, a sheet of formable metal is preheated to a temperature at which it can be stretched by a pressurized working gas against a forming surface of a heated forming tool. The sheet is then gripped around its edges by a binder apparatus surrounding the heated forming tool and thereafter a pressurized gas is applied to one side of the sheet to stretch the sheet and push an opposite side of the sheet into conformance with the forming surface of the heated forming tool. Often the pressure of the working gas is continually increased during the stretch forming in accordance with a pressurizing schedule. The sheet is thus permanently deformed, the gas vented, and the formed sheet removed from the heated forming tool.
Even though highly formable sheet metal alloys are used, it is sometimes found that a particular product shape cannot be obtained in a single hot stretch forming step without tearing or otherwise damaging the sheet metal. For example, certain automotive vehicle body panels cannot be reliably formed in a single hot stretch forming step even with a superplastically formable material such as fine grain AA5083, a magnesium and manganese containing aluminum alloy. In such a situation it is often possible to form the final product shape in two or more forming steps. The forming characteristics of the sheet material are considered in a plan to transform a flat or simply curved blank of suitable thickness and shape to the desired product configuration in two stretching steps. To this end, sophisticated double-action forming tools have been developed for preforming and final shape forming of a sheet metal workpiece using two forming tool halves in a single press. Such double-action forming tools typically operate in two stages. The first stage is a preforming stage for eliminating fold formation, and for creating necessary lengths of line and relatively uniform panel thickness distribution. The preform stage accomplishes a major portion of the stretching and elongation of the sheet in forming the sheet toward its final part shape. The finish stage completes bends and recessed corners and defines a final detailed shape of the sheet metal part.
In the preform stage either a punch tool or the pressure of a suitable working gas, such as air or nitrogen, is used to push against one side of the sheet and stretch it against a hot preform tool surface. Then gas pressure is applied to the opposite side of the sheet to stretch it in the opposite direction against a hot finish form tool. Thus, the necessary elongation lines or stretch directions in the sheet to form the part are predetermined. A substantial part of the elongation is accomplished in the preform step and is introduced nearly evenly over the preform shape. The final elongation is accomplished by forcing the preformed sheet away from the preform tool against the shaping surfaces of the finish-form tool.
The double-action stretch forming process is efficient in its utilization of a single press with upper and lower forming tools to transform a blank into a final product shape. However, the time required for the two stage forming steps limits the output of a single press. In order to produce more finished panels or other parts by such a practice, more presses with double action tooling are required and such manufacturing equipment is relatively complex and expensive. Accordingly, there is a need to increase the throughput of hot blow-forming operations for automotive body panels and other sheet metal parts while using less expensive tooling and presses.